This invention relates to chimeric proteins capable of simultaneously binding and inhibiting factor VIIa/tissue factor complex (factor VIIa/TF complex) and factor Xa, expression vectors coding for the proteins of the invention, host cells transformed with the expression vectors, methods for producing the proteins, pharmaceutical compositions containing the proteins, methods of treatment or prevention of septic shock using the proteins, methods of inhibiting coagulation disorders and monoclonal antibodies against the proteins.
Human Tissue Factor Pathway Inhibitor (TFPI) is a plasma protease inhibitor. Based on homology study, TFPI appears to be a member of the Kunitz-type basic protease inhibitor gene superfamily. TFPI functions in at least two ways: 1) Inhibition of the catalytic activity of factor VIIa/TF complex and 2) By binding to the active site of factor Xa. The primary sequence of TFPI, deduced from its cDNA sequence, indicates that the protein contains three Kunitz-type domains. The first of these, Kunitz-type domain 1, is believed to be required for the efficient binding to and inhibition of factor VIIa/TF complex, which is enhanced by the presence of the second Kunitz-type domain, Kunitz-type domain 2. Kunitz-type domain 2 is required for the efficient binding to and inhibition of factor Xa by TFPI. The function of the third Kunitz-type domain, Kunitz-type domain 3, is unknown. TFPI has no known enzymatic activity and probably inhibits the activity of protease targets in a stoichiometric manner, namely, binding of one Kunitz-type domain to the active site of one protease molecule. TFPI is also known as Lipoprotein Associated Coagulation Inhibitor (LACI), tissue factor inhibitor (TFI) and extrinsic pathway inhibitor (EPI).
Mature TFPI is a polypeptide of about 276 amino acids in length with a negatively charged amino terminal end and a positively charged carboxyl terminal end. The C-terminal tail (i.e., the sequence following the last cysteine residue of Kunitz-type domain 3) is highly basic and is believed to aid in the localization of TFPI to cell surfaces by binding to glycosaminoglycan (including heparin) or phospholipids found on cell surfaces. This cell surface localization property is believed to be important for full anticoagulant activity and for optimal inhibition of factor Xa TFPI contains 18 cysteine residues and forms 9 disulfide bridges when correctly folded. The primary sequence contains three Asn-X-Ser/Thr N-linked glycosylation consensus sites with asparagine residues at positions 145, 196, and 256. The carbohydrate component of mature TFPI is approximately 30% of the mass of the protein. Data from proteolytic mapping and mass spectral data imply that the native carbohydrate moieties are heterogeneous. Native TFPI is also found to be phosphorylated at the serine residue at position 2 of the protein to varying degrees. The role of phosphorylation at Ser-2 in TFPI function has yet to be elucidated.
Recently, another protein with a high degree of structural and functional similarity to TFPI has been identified, as described in Sprecher et al. Proc. Natl. Acad Sci. U.S.A. 91:3353-3357 (1994). The predicted secondary structure of this 213 amino acid residue protein, called TFPI-2, is virtually identical to TFPI having three Kunitz-type domains, 9 disulfide bridges, an acidic amino terminus and a basic carboxy terminus. The three Kunitz-type domains of TFPI-2 exhibit 43%, 35% and 53% primary sequence identity with TFPI Kunitz-type domains 1, 2, and 3, respectively. Compared with TFPI, recombinant TFPI-2 strongly inhibits the amidolytic activity of factor VIIa/TF complex and weakly inhibits factor Xa activity. TFPI-2 is reported to bind with greater affinity to factor VIIa/TF complex than does TFPI, whereas TFPI binds to factor Xa with greater affinity than does TFPI-2.
The presumed P1-reactive site in Kunitz-type domain 1 of TFPI-2 is arginine, as contrasted with lysine in TFPI. The P1-reactive site in Kunitz-domain 2 of TFPI-2 is glutamate, as contrasted with arginine in TFPI. Also, the Kunitz-type domain 2 of TFPI-2 contains two additional amino acid residues between the fourth and fifth cysteine residues. The spacer region between Kunitz-type domains 1 and 2 in TFPI-2 is much shorter than the corresponding TFPI spacer region. One or more of these differences may result in the different affinities of the two proteins for factor VIIa/TF complex and Xa.
TFPI has been shown to prevent mortality in a lethal Escherichia coli (E. coli) septic shock baboon model. Creasey et al, J. Clin. Invest. 91:2850-2860 (1993). Administration of TFPI at 6 mg/kg body weight shortly after infusion of a lethal dose of E. coli resulted in survival in all five TFPI-treated animals with significant improvement in quality of life, compared with a mean survival time for the five control animals of 39.9 hours. The administration of TFPI also resulted in significant attenuation of the coagulation response, of various measures of cell injury and significant reduction in pathology normally observed in E. coli sepsis target organs, including kidneys, adrenal glands, and lungs. Due to its clot-inhibiting properties, TFPI may also be used to prevent problems associated with thrombosis and clotting such as during microvascular surgery. For example, U.S. Pat. No. 5,276,015 discloses the use of TFPI in a method for reducing thrombogenicity of microvascular anastomoses wherein TFPI is administered at the site of the microvascular anastomoses contemporaneously with microvascular reconstruction.
TFPI has been isolated from human plasma and from human tissue culture cells, including HepG2, Chang liver and SK hepatoma cells. Recombinant TFPI has been expressed in mouse C, 127 cells, baby hamster kidney cells, Chinese hamster ovary cells and human SK hepatoma cells. Recombinant TFPI from the mouse C127 cells has been shown in animal models to inhibit tissue-factor induced coagulation. A non-glycosylated form of recombinant TFPI has also been produced and isolated from Escherichia coli (E. coli) cells as disclosed in U.S. Pat. No. 5,212,091. This form of TFPI has been shown to be active in the inhibition of bovine factor Xa and in the inhibition of human tissue factor-induced coagulation in plasma. In some assays, the E. coil-produced TFPI has been shown to be more active than TFPI derived from SK hepatoma cells. Methods have been disclosed for purification of recombinant TFPI from yeast cell culture medium, such as in Petersen et al, J. Biol. Chem. 18:13344-13351 (1993). Truncated forms of recombinant TFPI have also been studied, as described in Hamamoto et al. J. Biol. Chem. 268:8704-8710 (1993) and Petersen et al. ibid.
Petersen et al. ibid have attempted to produce TFPI and variants of TFPI, including: 1) variants of TFPI in which the complete C-terminal one third of the polypeptide, including Kunitz-type domain 3, was deleted; 2) variants of TFPI in which just Kunitz-type domain 3 was deleted; and 3) variants of TFPI in which the basic portion of the peptide, C-terminal to Kunitz-type domain 3, were deleted. They found that high yields were obtained only with the first variant. This variant was heterogeneously glycosylated, and its anti-coagulant activity was 5-50 fold lower than full-length TFPI obtained from mammalian cells.
A need exists, therefore, for a method to produce in high yield, a protein molecule that possesses at least the equivalent, if not enhanced, anticoagulant and other activities, of TFPI and that has reduced glycosylated moieties that would in turn result in reduced immunogenicity of the protein upon administration to a mammal.